The invention is directed to fiberglass power boats of the type propelled by transom mounted, outboard propulsion units; and more specifically to a transom reinforcement therefor.
Transom mounted propulsion units, and particularly those rated at 150 horsepower and above, impose severe structural stresses on the transom both as a function of their own weight (upwards of 400-450 pounds) and the changing torques applied to the transom propulsion unit transmits its thrust to the boat via the transom at varying speeds from start-up to more than 75 miles per hour.
The relatively long, upper transom edge spanning the splash well is particularly vulnerable to transverse flexure as thrust variations are transmitted thereto via the centrally mounted propulsion unit which is, essentially, "hung" from adjacent the upper edge thereof via its upper mounting holes. Similarly, the striking of underwater obstacles such as stumps or the like impose high impact torques which are transmitted to the upper transom edge. Those transverse flexural stresses that can be absorbed by the transom are transferred to the hull sides at opposite ends of the transom reinforcement frequently resulting, after long term use, in fracture at the splash well corners. In the case of violent impacts, such as by an underwater obstacle, the transom fractures centrally and the propulsion unit is literally torn from its lower mounting holes.
Fiberglass power boat transoms are, conventionally, reinforced by fiberglass encased plywood; the plywood being designed to provide structural strength for propulsion unit support and sufficient flexibility to absorb normal propulsion unit induced vibrations without fracturing. The thickness of the plywood reinforcement may vary from less than an inch to upwards of two inches depending, primarily, on the weight and horsepower rating of the propulsion unit to be supported. A secondary consideration in the choice of plywood thickness is a natural tendency to "overbuild" the engineering design to compensate for known variants in wood strengths as among different lots. Major disadvantages in the use of plywood reinforcements are the inherent weakness of the material in resisting transverse flexion and its susceptibility to rot; each of these disadvantages, after prolonged use, becoming more pronounced as a function of the other.
Although the conventional plywood transom reinforcement is encased in fiberglass, the transom is traversed by a number of through holes exposing the plywood. These through holes necessarily include the propulsion unit mounting holes and, optionally, drain and/or live well openings. The through holes provide moisture ingress paths that can rot the plywood when the through hole seals are defective or become defective through use. Exemplary of the latter is vibration induced seal displacement which, in turn, is a function of the weight, horsepower and use conditions of the supported propulsion unit producing transverse transom flexure. The problem of transom rot has increased in recent years as manufacturers have upped horsepower ratings to mcet consumer demand for higher performance which, in turn, increases the liklihood of transom fracture even under conservative operating conditions after the transom reinforcement has begun to rot.
The primary object of the invention is to provide a unit handled subassembly constituting a transom reinforcement that can be assembled and glassed as readily as a plywood reinforcement but which is many times stronger than wood and not susceptible to rot.